1. Field of the Invention
The present invention relates to a radiotherapy treatment planning system, so as to calculate the absorbed dose in an object to be irradiated, at radiotherapy with a medical linac and a cobalt-60 .gamma.-ray irradiating system.
2. Description of the Prior Art
FIG. 1 is an explanatory view to represent how to irradiate a lesion of humans (an object to be irradiated) with X ray in order to perform X-ray treatment. In the figure, the numerical FIG. 30 represents an X-ray source (radiation source); 32, a patient (human body); 34, a lesion of the patient; 36, the X-ray beam width.
As is shown in FIG. 2, the most part of X-ray 38 incident to the human body 32 is scattered by Compton scattering. That is, X-ray 38 bombards the electrons in the atoms of the human body 32 (water holds its most part), and X-ray itself is concurrently scattered and becomes a scattered beam 42. Comparing with the energy of the initial X ray, the scattered beam 42 has a reduced energy, by the energy which is given to the electrons, leading to a phenomenon to lengthen the wave length. The scattered angle .beta. of the electron beam 40 is then 90.degree. or less.
The absorbed dose of the lesion 34 from X-ray irradiation (referred to as "dose" hereinafter) is in proportion with the number of ion pairs generated when the electron beam 40 passes the lesion 34. As is shown in FIG. 3, the process for the electron beam 40 incident to an observational region 44 in the lesion 34 consists of a direct process in which the electrons through a first Compton scattering is directly incident to the observational region 44, a first scattering in which the electrons generated at a second scattering of X ray are incident to the observational region 44, and a multiple scattering in which the electrons generated at more than triple scattering are incident to the observational region 44.
The treatment of a patient 32 with X ray 38, requires the information of the dose of a lesion 34. This is derived from the need to determine the dose of X ray 38 so as to cause necessary damage on the lesion 34. Therefore, there is a demand for a radiotherapy treatment planning system to execute dose calculation. For conventional radiotherapy treatment planning systems, the dose calculation has been performed by employing Monte Carlo technique, convolution method, delta-volumetric method, equivalent tissue air ratio (ETAR) technique, power law method, scatter air ratio technique or RTAR technique.
Monte Carlo technique, a process in which the three-dimensional calculation without approximation is iteratively carried out by means of simulation until a sufficient precision can be achieved, proposes the most accurate dose distribution, but it takes eight hours or longer even if 1MIPS computer is employed as a radiotherapy treatment planning system. In order to shorten such calculation time, according to the convolution method, the distribution of dose contribution to other points due to the occurrence of scattering at one point is preliminarily calculated, which calculation is then applied to the entire X-ray irradiated points to add up the individual distributions together, leading to the determination of dose distribution. According to delta-volumetric method, concerning a region of interest, X ray is divided into direct ray, a first scattered beam and other scattered beams, which are then integrally calculated in three dimension. On the contrary, by ETAR technique, the three-dimensional effects of scattered beam are introduced into a two-dimensional equivalent plane to perform two-dimensional integration. As is shown in FIG. 4, the power law method is a correction method applied for the case in which there are heterogeneous layers of densities .rho..sub.1 and .rho..sub.2 in a depth direction. The symbol 46 herein represents the observational point in the observational region 44. RTAR technique is a method utilizing the evidence that the absorbed dose becomes identical if the ratio of an area in an irradiated field to a circumferential length is identical, when viewed from the side of a source 30. SAR technique is a method searching SAR table, after converting the contribution of scattered beam into an equivalent irradiation field radius, based on the form of an irradiated field.
The radiotherapy treatment planning systems utilizing each of the methods described above are generally realized by employing computer programs. In performing each calculation, tomographic images by X-ray CT are widely used. From the density of the CT images, the electron density corresponding to the X-ray energy is calculated for use.
The radiotherapy treatment planning systems utilizing each of the methods mentioned hereinabove are described in detail in "Computer Application in Radiation Therapy Treatment Planning", Vol. 1, No. 2, James A. Purdy, 1983.
Since the conventional radiotherapy treatment planning systems are in such structure as has been described above, they have such problems as follows; those determining the contribution of scattered beam by three-dimensional calculation take too long calculation time to be applied in practical radiotherapy treatment; those determining the contribution of scattered beam by two-dimensional calculation cannot sufficiently compensate the heterogeneity in electron density and the deformation of an irradiated field, so that there has been suggested a problem about them such that there cannot be performed under all conceivable conditions the dose calculation to meet the ICRU (International Commission on Radiation Units and Measurements) recommendation, namely, that the error in an applied dose should be suppressed within 5% or less. In case that the ETAR method has been used, for example, the error exceeding 7% has occurred in some cases.
FIG. 5 shows a real-time dose distribution arithmetic and display system in the conventional radiotherapy treatment planning systems. In FIG. 5, the FIG. 101 represents a displacement input unit comprising a truck ball or a rotary switch, provided with a required potentiometer to transform the input from an operator to the system, into an electric signal corresponding to the input. 102 is a main arithmetic unit, comprising a digital computer of a required scale. 103 is a high-speed arithmetic unit comprising an array processor, to execute the required calculation of dose distribution at a high velocity. 104 is an image memory, to memorize the calculated results of dose distribution with the high-speed arithmetic unit 103, in order to display the results on desirable images. 105 is a character-display CRT and 106 is a key board, both of which perform the necessary functions as man-machine interface between an operator and the system. 107 is a graphic CRT, to display on its scope the contents stored in the image memory 104.
FIG. 6 is a flow chart to explain the operation of the system.
The operation of the above system will be now explained with reference to FIGS. 5 and 6.
A first step S400, push and press motion of a certain suitable start key (not shown in the figure) or the like will make start the real-time dose distribution arithmetic and display system.
At next step S401, the key board 106 as a man-machine interface proposes a preset arithmetic condition (arithmetic matrix, arithmetic precision, computing time and the like) through an operator. As an arithmetic precision, for example, either one of high-precision calculation and simple calculation is then selected. At step S402 following the above step, it is judged whether high-precision calculation is selected or not. When the result of such judgment is YES, the displacement is input at step S408, sequentially followed by the transformation of the displacement into arithmetic parameters (S409), the calculation based on a preset formula with high precision (S410) and the display of the results of such calculation (S411). The operation from steps S408 to S411 is carried out as follows. That is, a preset displacement is input through a displacement input unit 101 by an operator, which is then transformed into a suitable therapeutic parameter such as angle of the body of a therapeutic system, form of an irradiated field, form of shielding blocks and the like, by the subsequent main arithmetic unit 102. Through the character display CRT 105 and the key board 106 as man-machine interface, being in connection with the main arithmetic unit 102, an operator preliminarily determines a preset arithmetic condition (arithmetic matrix, arithmetic precision, computing time and the like). The high-speed arithmetic unit 103 executes the required calculation by using the preset arithmetic formula for calculating dose distribution (a high-precision formula herein), on the basis of the arithmetic condition described above. The dose distribution data obtained as the results of the calculation is loaded into the image memory 104 at the subsequent step. By executing such operation on real time, the dose distribution based on the preset parameters is displayed on the graphic CRT 107. After the display is done, the initial step S401 is resumed.
Alternatively, when the result of the judgment at step S402 is NO, displacement is input at step S403, followed by the transformation of the displacement into calculating parameters (S404), the calculation based on a preset simple formula (S405) and the display of the results of such calculation (S406). At the subsequent step (S407), the judgment is done concerning whether or not real-time processing is completed. When the result of the judgement is NO, return to step S403 and then the processing of the following steps is repeated. On the contrary, when the result of the judgment at the above step S407 is YES, return to the initial step S401. The operation from the steps S403 to S406 is the same as the operation from the aforementioned steps S408 to S411, except the calculation by the simple formula. The detailed explanation is therefore not recited herein.
In the conventional real-time arithmetic result display system in such structure as described above, the arithmetic condition is preliminarily set by an operator, and the system realizes real-time (calculation) display based on a preset displacement input. In the conventional system, real-time display using an arithmetic formula with a high speed but with a low precision (in other words, for example, smaller arithmetic matrix) is generally done, while an operator searches the optimum parameters while appropriately changing input displacement. Consequently, real-time (calculation) display with a high-precision formula (in other words, larger arithmetic matrix) is executed, by using the optimum parameters searched. There has been suggested such a problem that it becomes extremely complex the work to optimize therapeutical parameters under a high-precision condition (in other words, larger arithmetic matrix) which is necessary for an operator.